A heat pipe is a heat transfer mechanism that can transfer a large amount of heat with a very small temperature differential between its hot portion (or higher temperature region) and its cold portion (or lower temperature region). Inside a heat pipe, at the hot portion, a working fluid is evaporated into a vapor which then flows through a vapor region within the heat pipe and subsequently condenses at the cold portion into a liquid. The liquid is moved through a liquid region within the heat pipe (e.g., by gravity and/or capillary action) back to the hot portion to be evaporated again, creating a heat transfer cycle. Because heat pipes contain no mechanical moving parts, they typically require very little maintenance. In addition, as a heat transfer mechanism, a heat pipe has much higher efficiency in transferring heat, and is a much better heat conductor, than an equivalent solid metal block (or pipe).
In more detail, a typical heat pipe is composed of a sealed housing (or pipe), often made of a material with high thermal conductivity such as copper or aluminum. The sealed pipe is evacuated to form a vacuum, and then a portion of the pipe is filled with a working fluid (or coolant). Due to the pressure inside the heat pipe that is near or below the vapor pressure of the fluid, some of the working fluid will be in the liquid phase and some will be in the gas phase.
A wick structure may be included within the pipe walls to exert a capillary pressure on the liquid phase portion of the working fluid to wick the liquid back to the hot portion. Use of a wick structure within the heat pipe results in higher effective thermal conductivity than solid materials, alone, for example even good solid thermal conductors such as graphite, might allow. A typical wick structure may consist of a foam material having random cell configurations, a sintered particle bed, a wire mesh, or a set of grooves. The wick structure may also include a network of arteries (an arterial wick structure) having a network of arteries configured to wick the liquid back to the hot portion while reducing or minimizing pressure drops in the heat pipe. While these arterial wick structures are optimized for heat pipes requiring higher performance where, for example, the working fluid must travel a long distance between hot portion and cold portion or a large axial heat flux is imposed, known arterial wick structures often experience blockages caused by vapor bubbles. More specifically, vapor bubbles often form in the arteries of arterial wick structures, causing arterial function to cease and greatly reduce the maximum heat flux of the heat pipe.
Moreover, arterial wick structures are more commonly found in one-dimensional or tubular type heat pipes having a generally cored or circular cross-section. These one-dimensional heat pipes typically include adequate space to accommodate the network of arteries of an arterial wick structure such that the arteries can be separated a distance from the interior walls of the heat pipe. Separating the arteries from the possibly hotter interior walls is necessary for reducing or minimizing the occurrence of vapor bubbles in the arteries, and, thus, minimizing or preventing a possible reduction in maximum heat flux through the heat pipe caused by these vapor bubbles. However, these same arterial wick structures are not as easily accommodated in planar pipes—which typically utilize homogenous wicks or vapor chambers—with little to no space to separate out the arteries from the interior walls of the heat pipe. As such, the risk of vapor bubbles nucleating in the arteries of planar heat pipes is greatly increased.
Additional considerations must also be made in using planar rather than one-dimensional heat pipes. For example, while all heat pipes must be designed to withstand pressure differentials between the working fluid and ambient pressure, one-dimensional heat pipe design can be more readily adjusted to account for increases in pressure, for example, by thickening the interior wall of the heat pipe. This increased interior wall thickness to handle a given pressure in a one-dimensional pipe would result in a more negligible increase in the overall mass or weight of the heat pipe, owing to the cylindrical or tubular geometry of the one-dimensional heat pipe. In contrast, planar heat pipes, having a much higher wall area to total volume ratio and a larger surface area that must resist the pressure differential, experience a much more significant change in weight or mass resulting from a comparable interior wall thickness increase. To account for this less desirable weight or mass increase in planar pipes requiring additional pressure-resistibility, rather than increase the interior wall thickness or in combination with a smaller thickness increase, mechanical members may be placed spanning the vapor region of the heat pipe. These mechanical members may be configured to add structural integrity to the heat pipe, preventing or minimizing a collapse or burst in the heat pipe. Addition of mechanical members, however, increases pressure loss of the vapor, thus reducing maximum heat flux through these areas of the heat pipe.
As such, there is a need for a heat pipe configured to transport heat with higher effective thermal conductivities than solid materials can transport, to transport heat in planar form factors, and to transport heat at higher heat fluxes than currently available.